Presently, the most widely used optical transmitter has a three-dimensional structure that houses LD for emitting transmission light to a cylindrical metal package, a monitor PD for monitoring LD output, a lens for condensing and causing the transmission light to be incident on optical fibers, and a cap for holding the lens.
FIGS. 1, 2, and 3 each show conventional optical transmitters. A vertical pole 2 is formed on a disk-like metallic stem 1. A semiconductor LD 4 on a submount 3 that generates a signal light beam is fixed sideways onto a side face of the pole 2. A monitor PD 5 is disposed just under the semiconductor LD. This is insulated with a submount 6 from the stem 1. Lead pins 7 through 9 are projecting from the bottom of the stem downward. The electrodes of the monitor PD 5 and the LD 4 are connected to the lead pins by wires 10 and 11. A lens 12 is disposed just above the semiconductor LD 4. This is a ball lens and is held by a cylindrical cap 13. An optical fiber 14 is disposed thereabove.
In practice, a cylindrical, thin ferrule holds the optical fiber 14, and a cylindrical metallic ferrule holder holds this ferrule. The metallic ferrule holder is soldered onto the upper surface of the stem 1. The metallic stem and the metallic ferrule holder constitute a metal package. The metal package is hermetically sealed and is filled with an inert gas.
Such an outer shell of the metal package is not illustrated in the drawings. The semiconductor LD 4 emits light from both end faces thereof. A front light 15 is signal light beam, and a rear light 16 is monitor light beam.
In the semiconductor LD 4, the ratio of the amount of the front light 15 to the rear light 16 is fixed. Therefore, the power average value of the transmission light (front light 15) of the semiconductor LD 4 can be kept always constant by receiving the rear light 16 of the semiconductor LD 4 using the monitor PD 5 and by controlling a drive circuit of the semiconductor LD 4 so that the monitor PD's photocurrent output may become constant. Since the light intensity of the LD varies over a long period of time, a monitor PD for monitoring it is indispensable for the optical transmitter.
Since the metal package has been used, the optical transmitters shown in each of FIGS. 1 through 3 have been good in hermetic sealing, and satisfactory in emission output and amount of monitoring light. However, since they have a three-dimensional structure, axis alignment has been necessary for positioning the cap 13 with the lens and the optical fiber. Additionally, since assembling may have been time consuming, assembly costs have been high. Further, since the lens for condensing light and the coaxial metal package have been used, there have been a limit in miniaturizing its size and reducing costs.
An optical communication device having a two-dimensional structure and not requiring axis alignment adjustment has been studied. This is called a surface-mountable optical communication device because a planar waveguide is installed on a substrate. This device has a two-dimensional structure, is small in size, and has a short optical path, and therefore no lens required, and so axis-alignment work requiring much time has become unnecessary. Additionally, there is a possibility that components and assembly costs can be reduced. Such an optical communication device may exactly meet the purposes of size and cost reductions. Since the LD light emitting a signal light beam is not allowed to vary even in case of the surface-mountable transmitter, the monitor PD must monitor the amount of the LD light.
If a PD performing edge illumination is used as the PD for monitoring the LD light, the LD and the monitor PD may be directly mounted on the substrate while adjusting their height. For example, there have been waveguide type PD or an edge-incident type PD whose end face being slanted. The height of the LD and the PD can be adjusted by mounting them on the substrate while directing their respective epitaxial layer surface toward the substrate. The rear light having horizontally emitted from the LD directly illuminates the end face of the PD, thereby making it possible to generate a monitor current in the PD.
However, these PDs are unique and so have been difficult to be manufactured at low cost. Further, if these PDs are used, only a part of the LD light enters the PD because an active area is too narrow. In short, disadvantageously, the coupling efficiency of the LD is low, and the amount of monitor light is slight. All that is needed for accurate monitoring is to enhance the responsivity of the PD or increase the amount of the rear LD light.
In order to achieve cost reductions, it is absolutely necessary to use, as a monitor PD, a rear- or top-incident PD, which can be easily manufactured. These have requirements of causing a light beam to enter from a rear or top face.
A possible method for horizontally disposing the LD and causing the light to enter from the rear or top face of the PD is considered to mount the PD sideways, for example.
In case of the top-incident PD, it is possible to set up a pole on the substrate and then mount the PD sideways on the side face of the pole. Thereby, the rear light (monitor light) of the LD can enter from above the top face of the PD at a right angle. Also, in case of the rear-incident PD, it is possible to set up a pole on the substrate and thereafter fix the rear-incident PD thereto so that the light to enter from the rear face thereof. However, in order to do so, there is a need to install the pole onto the substrate, thus having complicated the structure of the substrate. Since light that has emitted from the LD radially spreads as a result of propagation, a lens for converging it may have been needed. Further, wiring patterns are complicated, and a quite number of parts and man-hours have been required, and, as a result, the advantages of the surface-mountable type may have been decreased. It is contrary to cost reductions to set up the pole on the substrate and complicate the device structure.
Under the situation where the LD is horizontally disposed, and the rear-incident PD that is placed horizontally on the substrate is used as a monitor PD, some measure is needed to guide the light of the LD to the rear face of the PD. For example,
(1) German Patent DE4313492C1 discloses a structure in which a V-groove is formed on a substrate surface just behind an LD mounted surface thereof directing its epitaxial layer toward the substrate, and a rear-incident PD is disposed as a monitor PD so as to step over the V-groove. According to this structure, the rear light of the LD is reflected by the V-groove, is thereafter directed upward, and reaches a detecting area of the PD from the rear face thereof. Here, the LD and the PD are coupled to each other merely by the V-groove having been cut between the LD and the PD. However, in such a surface-mountable LD module (optical transmitter), a part of the LD rear light travelling toward the upper half does not enter the PD. That is, a disadvantage resides in the fact that travelling light toward the PD is less than half the rear light of the LD, and therefore coupling efficiency is low.
(2) Japanese Unexamined Patent Application Laid Open No. 1980-341062 discloses an LD module that an LD and a PD are horizontally mounted on an intricately shaped substrate having grooves, a chamfer, and stepped parts, and the rear light of the LD is guided to the rear-incident PD. FIG. 4 shows its structure. An Si substrate 17 is a single-crystal silicon (Si)substrate, and has a complicated shape. A narrow platform is provided at the central part, and a triangular groove 18 is formed behind this. The back of the substrate behind the groove 18 is a diagonally cut chamfer 19. A light-emitting element (LD) 20 is horizontally mounted on the narrow platform at the center. A high rear-tall portion 21 is provided just above the chamfer 19.
The rear-tall portion 21 is flat, on which a rear-incident monitor PD 22 is mounted. The rear half surface of the sectional triangular groove 18 is coated with an anti-reflection (AR) coating 23. A metal coating (reflecting mirror) 24 is formed onto the chamfer 19. A V-groove has been cut in front of the LD 20, and an optical fiber 25 is inserted and fixed there. The LD 20 (light emitting element) emits front light (signal light) and rear light (monitor light). The power ratio thereof is fixed.
The signal light 29 (front light) enters the optical fiber 25, and is propagated therethrough. The monitor light 28 (rear light) travels through the groove 18 downward, passes through the AR coating 23, enters and travels through the substrate, then being reflected by the metal coating 24 of the back, then enters the rear face of the monitor PD 22, and is detected thereby. The magnitude of the signal light can be detected by monitoring the light received by the PD 22. Since the monitor PD can detect the decrease in the LD light due to aging change, a current for driving the LD may be increased proportionately with the decrease, and thereby the strength of the LD signal light being kept constant. This was a novel idea in that the monitor light can travel through the Si substrate.
Si is obscure to visible light, but is almost transparent to signal light of 1.3 μm or 1.55 μm, and therefore can allow the rear light of the LD 20 to pass through the interior of the Si substrate. Thus, such a structured surface-mountable module had not been theretofore found. Moreover, it was a contrivance that had surprised a person skilled in the art, as it is the method of diagonally chamfering the bottom surface of the substrate 17 (see reference character 19), then processing the mirror 24 (metal coating) thereby reflecting the monitor light upward. Thus, the rear light 28 of the LD can be guided to the rear face of the PD horizontally mounted on the substrate. The excellent idea made in this proposal resides in the fact that light can be guided from the LD to the PD both chips horizontally mounted on the substrate.
This method may be a promising candidate, because it is possible to successively mount an optical fiber, a semiconductor LD, and a monitor PD on a small bench utilizing surface mount technology, with a lens not needed, and there is a possibility of enabling size and cost reductions.
However, according to this method, the Si bench must be subjected to groove and chamfer processing, and such processing of the Si bench is complicated. It is disadvantage of this method. Since the height of the rear-tall portion 21 on which the PD is placed is greater than that of an intermediate part on which the LD 20 is placed, steps having such a level difference must be formed. Further, the groove 18 must be formed in the intermediate part, and the chamfer 19 and a mirror 24 must be formed on the back of the rear substrate. Especially, oblique processing of the rear chamfer 19 and processing of the metallic reflection coating 24 increases man-hours. The difficult processing leads to a great rise in the manufacturing costs of the module.
There is still another disadvantage in addition to the aforementioned. It is that the coupling efficiency between the LD 20 and the PD 22 is lower than that of Prior Art (1). In other words, the rear light 28 of the LD 20 only partly enters the monitor PD 22.
This can be understood from extremely simple consideration in terms of geometrical optics. The emission point of the rear light of LD 20 is designated as L2. The base of the rear-incident PD 22 is designated as CG, and the middle point of CG as N. The side of the mirror is designated as EF. The downward angle of the rear light 28 that has emitted from the LD 20 forming an optical axis is designated as φ. The downward angle φ is a variable. The point where a part of the rear light striking the middle point N of the PD having been reflected by the mirror 24 is designated as M. M is situated at almost the center of EF, but is not the middle point. The slanting angle of the mirror surface is designated as β. β is a constant. In the FIG. 4, light striking the middle point N of the PD is shown so as to be perpendicular to the bottom surface of the PD. In the intermediate point M of the mirror, an x-axis is set sideways, and a y-axis lengthways. The horizontal distance between the emission point L2 of the rear light of the LD 20 and the intermediate point M of the mirror is designated as s, and the vertical distance therebetween as h. The distance between the mirror intermediate point M and the middle point N of the PD is designated as k. k is greater than h (k>h). The mirror surface having a slanting angle of β is expressed by the following linear equation.x sin β−y cos β=0  (1)The back surface of the PD is expressed by the equation (y=k).
The light beam emitted from the LD at an emission angle of “φ0=cot−1(s/h)” is reflected at the mirror intermediate point M(0,0), and strikes the middle point N(0,k) of the rear face of the PD. Presumably, when φ=0, the light beam is maximum and shows the Gaussian distribution therearound. However, the light beam has a certain degree of strength even in the vicinity of φ0. A reflection angle at the point M is equal to the slanting angle β, and an incidence angle at the point,where M is π/2−(φ0+β). Accordingly, the following equation is obtained if assumed these being equal to each other.2β=π/2−φ0  (2)The emission angle φ of the LD light where the light beam striking an arbitrary point (x, k) of the bottom surface of the PD is given by following equations.φ−φ0=tan−1(x/(k+q))  (3)q2=h2+s2  (4)It is understood that, since the distance “k+q” is long, only light falling within the narrow range of the emission angle φ can enter the PD. Equation (3) can be approximated like “φ−φ0≈x/(k+q)”. Then, the range of the emission angle φ detected by the PD is:|φ−φ0|≦D/{2(k+q)}  (5)where D is an effective diameter of the detecting area in the PD.Let the effective diameter D of the PD be 200 μm, and k+q=3000 μm. Then, |φ−φ0| is 0.032 radians (=2 degrees). If φ0=10 degrees, for example, then only the rear light whose emission angle being 8 through 12 degrees enters the PD. This indicates that the longitudinal light beam divergence of the LD is approximately 20 trough 30 degrees, out of which only light beams travelling in extremely limited directions can enter the PD.Above all the strongest LD light in the vicinity of zero (0) degrees can not be caught. Therefore, most of the rear light of the LD becomes useless. It is to be understood that the coupling efficiency between the PD and the LD is low.
Prior Art (2) may be dominated by a preconceived idea that both the semiconductor LD and the monitor PD might be horizontally mounted. If the chips are horizontal to the substrate surface, chip mounting and wire bonding can be easily performed. Therefore, the structure shown in FIG. 4 may have been inevitably employed.
However, in order to make more use of the surface mount technology further, it is preferable to dispose a simpler monitor PD. In other words, it is preferable to employ a simple structure without a plurality of grooves having not been cut, and a mirror nor an antireflection coating having not been formed. In addition, such a structure is expected as most of the rear light of the LD can strike the PD by enhancing the coupling efficiency between LD and PD.
(3) Japanese Unexamined Patent Application Laid Open No. 1997-26529 discloses a structure that the signal light beam of an optical fiber is received by a rear-incident PD with a lens having been slantingly fixed, as shown in FIG. 9 of this publication. However, this is not a light receiving monitor PD but a signal light receiving PD. Accordingly, it is necessary that the response speed be high, and it is impossible to enlarge an active area for it. FIG. 9 of this publication shows an active area whose radius is 30 μm. A lens is indispensable to be provided in order to gather light in the small active area. However, it is difficult to fix the lens so as to adjust the focus of the lens at the active area of the PD. Additionally, since the optical fiber cannot be stably positioned, axis alignment is needed. However, the position of the optical fiber is often deviated despite the axis alignment. The structure disclosed has not be reduce costs. In the optical fiber, its polarization plane rotates because of mechanical shock, temperature variation, and pressure change. A slanting face incidence method, heavily polarization-plane dependent, has a disadvantage in that its responsivity tremendously varies due to the influence of the polarization plane rotation.